Organic ultralong room temperature phosphorescence (RTP), or organic afterglow, is a unique phenomenon, gaining widespread attention due to its far‐reaching application potential and fundamental interest. Here, two laterally expanded 9,10‐dimesityl‐dihydro‐9,10‐diboraanthracene (DBA) derivatives are demonstrated as excellent afterglow materials for red and blue‐green light emission, which is traced back to persistent thermally activated delayed fluorescence and RTP. The lateral substitution of polycyclic DBA scaffold, together with weak transversal electron‐donating mesityl groups, ensures the optimal molecular properties for (reverse) intersystem crossing and long‐lived triplet states in a rigid poly(methyl methacrylate) matrix. The achieved afterglow emission quantum yields of up to 3 % and 15 %, afterglow lifetimes up to 0.8 s and 3.2 s and afterglow durations up to 5 s and 25 s (for red and blue‐green emitters, respectively) are attributed to the properties of single molecules.

Seven new derivatives of phenanthro[9,10-d]imidazole having differenet substituents at the 1st and the 2nd positions of the phenanthroimidazole moiety were synthesized and characterized. The comparative study of their properties was performed employing thermal, optical, electrochemical and photoelectrical measurements. The properties of the newly synthesized compounds were compared with those of earlier reported derivatives of phenanthroimidazole and several interesting new findings were disclosed. Density functional theory calculations accompanied by optical spectroscopy measurements have shown the possibility of tuning the emission properties (excited-stated decay rate, fluorescence quantum yield, etc.) of phenanthro[9,10-d]imidazole derivatives via attachment of different substituents to the 1st and the 2nd positions. The most polar and bulky substituents linked to the 2nd position were found to have the greatest impact on the emissive properties of compounds causing (i) fluorescence quantum yield enhancement of dilute liquid and solid solutions (up to 97%), (ii) suppression of intramolecular torsion-induced nonradiative excited-state relaxation in rigid polymer films as well as (iii) inhibition of aggregation-promoted emission quenching in the neat films. Most of the studied compunds exhibited ambipolar charge transport character with comparable drift mobilities of holes and electrons. The highest hole and electron mobilities approaching 10⁻⁴ cm² V⁻¹ s⁻¹ were observed for the derivative having a triphenylamino group at the 1st position of the imidazole ring and the phenyl group at the 2nd position. The estimated triplet energies of phenanthro[9,10-d]imidazole compounds were found to be in the range of 2.4–2.6 eV, which is sufficiently high to ensure effective energy transfer to yellow/red emitters.

Efficient TADF relies on effective reverse intersystem crossing (rISC) from triplet to singlet manifold, and therefore the transition rate associated with this process must outcompete non-radiative decay rate of the triplets. Herein the issue of non-radiative triplet quenching in benzophenone-derived compounds frequently employed as blue TADF emitters in OLEDs is addressed and an effective way to suppress this quenching is proposed. TADF emitters designed for this study were based on phenone-derived electron-accepting units linked with multiple carbazolyl donors to deliver similar and large rISC rates (k_rISC ∼ 4 × 10⁶ s⁻¹), yet significantly different non-radiative triplet quenching rates (k^T _nr). We found that substitution of a loose phenyl moiety by a methoxy group in benzophenone suppresses k^T _nr by one order of magnitude (from 3 × 10⁶ s⁻¹ down to 0.3 × 10⁶ s⁻¹), while leaving rISC rate k_rISC almost unaffected. A 10 times enlarged ratio k_rISC/k^T _nr in 5tCzMeB as compared to that in 5tCzBP emitter suffices to retrieve the vast majority of triplet population via the rISC resulting in PL quantum yields of 0.99 and 0.58 in doped DPEPO and neat films, respectively. Sky-blue TADF OLEDs with doped and non-doped emissive layers based on 5tCzMeB demonstrated impressive maximum external quantum efficiency (EQE) values of 24.6% and 13.4%, respectively, whereas at 100 cd m⁻² the devices exhibited 16.5% and 7.7% EQEs, respectively.

Narrow-band deep-blue (emission peak < 460 nm) TADF emitters are in demand for commercial OLED display applications, yet the development of efficient emitters with low efficiency roll-off is very challenging. To address this issue, herein, we studied carbazole–naphthyridine (donor–acceptor)-based blue-emitting TADF compounds, which were designed by using both the H-bonding and sterically controlled charge-transfer (CT) interactions between D and A units. Methyl substitution employed at the first position of t-butyl-carbazole donors was found to affect CT strength and consequently the TADF properties of the studied compounds, enabling a significant reduction of delayed fluorescence lifetime (down to 3.1 μs) and enhancement of reverse intersystem crossing rate (up to 10⁶ s⁻¹). The naphthyridines were demonstrated to hold great potential as deep-blue TADF emitters suitable for both vacuum- and solution-processed TADF OLEDs. The optimized devices with 7 wt% naphthyridine emitter in a weakly polar mCP host delivered external quantum efficiencies (EQEs) of up to ∼17.6% and ∼13.5% for vacuum- and solution-processed OLEDs, respectively. Unsubstituted naphthyridine exhibited deep-blue (λ_max < 460 nm) and narrow-band (FWHM = 66 nm) electroluminescence, whereas the more twisted methyl-substituted compound expressed broader band (FWHM > 80 nm) sky-blue (λ_max ≈ 480 nm) emission. The demonstrated emitters are among the best-performing conventional D–A-type blue/deep-blue TADF emitters in terms of EQE and efficiency roll-off properties of their devices.

Singlet fission is detrimental to NIR-to-vis photon upconversion in the solid rubrene (Rub) films, as it diminishes photoluminescence efficiency. Previous studies have shown that thermally activated triplet energy transport drives singlet fission with nearly 100% efficiency in closely packed Rub crystals. Here, we examine triplet separation and recombination as a function of intermolecular distance in the crystalline films of Rub and the t-butyl substituted rubrene (tBRub) derivative. The increased intermolecular distance and altered molecular packing in tBRub films cause suppressed singlet dissociation into free triplets due to slower triplet energy transport. It was found that the formation of correlated triplet pairs ¹(TT) and partial triplet separation ¹(T···T) occurs in both Rub and tBRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in tBRub, geminate triplet annihilation of ¹(T···T) outcompetes dissociation into free triplets, resulting in emission from the ¹(TT) state. Essentially, increasing intermolecular distance up to a certain point (a sweet spot) is a good strategy for suppressing singlet fission and retaining triplet-triplet annihilation properties.